Conventional ultrasound scanners create two-dimensional B-mode images of tissue in which the brightness of a pixel is based on the intensity of the echo return. Alternatively, in a color flow imaging mode, the movement of fluid (e.g., blood) or tissue can be imaged. Measurement of blood flow in the heart and vessels using the Doppler effect is well known. The phase shift of backscattered ultrasound waves may be used to measure the velocity of the backscatterers from tissue or blood. The Doppler shift may be displayed using different colors to represent speed and direction of flow. In power Doppler imaging, the power contained in the returned Doppler signal is displayed. Although the following disclosure refers predominantly to B-mode imaging for the sake of brevity, the present invention applies to any mode of ultrasound imaging.
Two-dimensional ultrasound images are often hard to interpret due to the inability of the observer to visualize the two-dimensional representation of the anatomy being scanned. In addition, it may not be possible to acquire the precise view needed to make a diagnosis due to probe geometry or poor access to the area of interest. However, if the ultrasound probe is swept over an area of interest and two-dimensional images are accumulated to form a three-dimensional data volume, the anatomy becomes much easier to visualize for both the trained and untrained observer. Also views which cannot be acquired due to probe geometry or poor access to the area of interest can be reconstructed from the three-dimensional data volume by constructing slices through the volume at the difficult to obtain angle.
In order to generate three-dimensional images, the imaging system computer can transform a source data volume retrieved from memory into an imaging plane data set. The successive transformations may involve a variety of projection techniques such as maximum, minimum, composite, surface or averaged projections made at angular increments, e.g., at 10.degree. intervals, within a range of angles, e.g., +90.degree. to -90.degree.. Each pixel in the projected image includes the transformed data derived by projection onto a given image plane.
In free-hand three-dimensional ultrasound scans, a transducer array (1D to 1.5D) is translated in the elevation direction to acquire a set of image planes through the anatomy of interest. These images can be stored in memory and later retrieved by the system computer for three-dimensional reconstruction. If the spacings between image frames (hereinafter "inter-slice spacings") are known, then the three-dimensional volume can be reconstructed with the correct aspect ratio between the out-of-plane and scan plane dimensions. If, however, the estimates of the inter-slice spacing are poor, significant geometric distortion of the three-dimensional object can result.
In the prior art, a variety of motion control and position-sensing methods have been proposed to control or track the elevational motion of the ultrasound probe respectively. However, these systems are often costly and cumbersome to use in a clinical environment. Therefore, to reconstruct a three-dimensional image with good resolution in the elevation direction, it is highly desirable to be able to estimate the scan plane displacements directly from the degree of speckle decorrelation between successive slices.
If the ultrasound probe is swept over an area of body and the slices of pixel data are stored in memory, a three-dimensional data volume can be acquired. The data volume can be used to form a three-dimensional view of the area of interest. The registration of these slices of pixel data is critical to the quality of the three-dimensional reconstruction. Since ultrasound probes are commonly held in the sonographer's hand and moved over the body in the course of an ultrasound examination, it would be desirable for the sonographer to acquire the three-dimensional data volume with a manual sweep. However, this makes the individual slices difficult to register.
A method and an apparatus, disclosed in U.S. patent application Ser. No. 09/045,780, for tracking scan plane motion in free-hand three-dimensional ultrasound scanning uses adaptive speckle correlation. The method employs a correlation index which adapts to different display dynamic range and post-processing filters. In particular, this method is based on computing the sum of absolute differences (SAD) between corresponding pixels in two kernels being correlated. The preferred method comprises the steps of choosing a kernel within each image frame for correlation calculations; rejecting duplicate image frames; measuring the degree of correlation between successive image frames; rejecting correlation estimates which may be associated with hand jitter and other artifacts; and computing the average frame-to-frame (i.e., inter-slice) spacing based on the average correlation estimate. These steps are performed by a host computer which interfaces with the cine memory. A major benefit of this image-based motion tracking technique is that it enables three-dimensional reconstruction with good geometric fidelity, without use of any external position-sensing device.
The foregoing method works well when the probe is moved in a linear fashion. However, if the probe is shifted in the X or Y plane or rotated about the X, Y or Z axis while being translated in the Z direction, a large error in the inter-slice distance estimate can be introduced. Thus, there is a need for a method which takes these shifts and rotations into account when estimating the inter-slice distance.